One of the goals of commercial agriculture is to increase the yield of crops. Yield increases have been obtained, for example, by reducing plant stature. By reducing the stature of plants, relatively less of the plant's biomass becomes involved in the production of stem and leaves and relatively more of the plant's biomass becomes involved in grain production. Furthermore, by reducing the stature of plants, the resistance of plants to lodging, i.e., falling over, is increased. Lodging is undesirable because the resultant flattened crop and secondary growth make harvesting difficult. Moreover, when plants lodge, the grain becomes dirty and wet and rots, thereby necessitating grain drying and causing delay in harvesting and loss of yield.
Throughout the history of agronomy, plant breeders have produced plants of reduced stature by selection and breeding, or by induction of mutation through irradiation or chemical means, as discussed in Dalrymple, D. G., DEVELOPMENT AND SPREAD OF HIGH-YIELDING RICE VARIETIES IN DEVELOPING COUNTRIES, Bureau for Science and Technology, Agency for International Development, Washington, D.C. (1986). The production of such plants of reduced stature, in accordance with the conventional methods, entails selection of appropriate parent stock, manipulation of the parent stock by hybridization or mutation, propagation of progenies, and further selection and breeding. Such processes are time-consuming and expensive.
Plant growth and development are also known to be regulated by the external application of plant growth regulators, such as hormones and other chemicals. Responses to such chemical applications, however, are variable, as discussed in Herbert, C. C., Growth Regulation in Cereals--Chance or Design?, in CHEMICAL MANIPULATION OF CROP GROWTH AND DEVELOPMENT, J. S. McLaren, ed., Butterworth Scientific (London 1982), pp. 315-327. Moreover, application of chemicals for purposes of regulating plant growth has to be performed in accordance with a precise schedule, without much allowance for flexibility.
The plants of reduced stature produced by conventional breeding means, such as those described above, are normally adapted to growth and cultivation under a specific set of environmental conditions. Because of the elaborate processes involved in breeding and adaptation, however, only a limited number of dwarf varieties of a particular crop are customarily adapted for cultivation in a given region. This often leads to planting of either a monoculture or a limited variety of crops by a given farmer. It is a concern among commercial farmers, therefore, that should disease strike the one or few varieties cultivated, the entire crop will be decimated. It would be desirable, therefore, to provide an easier, less expensive and less time-consuming method of producing a plant of reduced stature so that more varieties of a crop can be planted at any given time.
Microorganisms have been known to affect plant growth and development. Certain types of microorganisms, such as hybrid agricultural-chemical-producing endosymbiotic microorganisms, colonize the interior of plants and provide useful agricultural chemicals, such as pesticides, to the plants. Certain microbial endophytes are capable of inducing enhanced resistance in a host to phytopathogens.
Furthermore, pathogenic strains of Clavibacter xyli that inhabit sugar cane and bermudagrass and cause stunting disease in these plants have been reported. In Davis, M. J. et al. [J. System. Bacteriol. 34:107-117 (1984)], ratoon stunting disease of sugarcane is attributed to infection by Clavibacter xyli subsp. xyli. The stunting effect of this infection was associated with significant yield losses. In Davis, M. J. and Augustin, B. J. [Plant Disease 68:1095-1097 (1984)], a bermudagrass stunting disease is attributed to Clavibacter xyli subsp. cynodontis. The diseased grass reportedly appeared as unsightly circular patches of chlorotic and dying grass. The bacterial species causing ratoon stunting disease and that causing bermudagrass stunting disease were determined to be distinct from each other because each was able to induce a disease condition in its natural host but not in the natural host of the other. Yet, on the other hand, both bacteria could be found growing in the xylem vessels of both hosts, as stated in Davis et al. (1984). Subspecies of Clavibacter xyli, therefore, have been known to cause disease only in their natural hosts.
In addition to bacterial cells, fungal endophytes have also been known to cause stunting in infected plants, particularly, in infected grasses as discussed in Clay, K., Mycol. Res. 92:1-12 (1989). Fungal endophytes, however, often produce toxic alkaloids that are not appropriate for agricultural crops. In particular, for purposes of producing plants that yield edible seeds such as rice, infection by fungal endophytes are associated with certain disadvantages. For example, plants that are infected with fungal endophytes can become sterile and not produce seeds at all, or the fungal spores and toxic materials produced by the fungal endophytes, such as alkaloids, may find their way into the seeds, thus making the seeds unsatisfactory as food sources. Hence, the use of fungal endophytes to produce a reduction in plant stature is not desirable.
The present inventors have recognized that there exists a need for an easy, inexpensive and efficient method of producing a plant of reduced stature, particularly, that of an agricultural crop. They have recognized that it would be desirable if a microorganism can be found that can be introduced into a nonnatural host to produce a plant of reduced stature and yet, would not produce any harmful or undesirable side effects associated with stunting due to a natural infection of such a microorganism in its natural host. Such a microorganism should not produce any toxic compounds such as alkaloids and should not sporulate or grow in the seeds of the host so as not to contaminate, for example, the seeds of cereal crops. Infection of plants by a microorganism that can produce the above-described effect would be an easy, efficient and inexpensive method of producing plants of reduced stature.